1. Field of the Invention
This invention relates to an image-forming apparatus comprising an electron source realized by arranging a plurality of electron-emitting devices and a method of manufacturing the same.
2. Related Background Art
CRTs have been widely used for image-forming apparatus for displaying images by means of electron beams.
In recent years, on the other hand, flat panel display apparatus utilizing liquid crystal have been replacing CRTs to some extent. However, they are accompanied by certain drawbacks including that they have to be provided with a back light because they are not of an emissive type and hence there exists a strong demand for emissive type display apparatus. While plasma displays have become commercially available as emissive type display apparatus, they are based on principles that are different from those of CRTs and can not fully compete with CRTs, at least currently, from the viewpoint of contrast, chromatic effects and other technological factors. Since an electron-emitting device appears to be very promising for preparing an electron source by arranging a plurality of such devices and an image-forming apparatus comprising such an electron source is expected as effective as CRT for light emitting effects, efforts have been made in the field of research and development of electron-emitting devices of the type under consideration.
For instance, the applicant of the present invention has made a number of proposals for an electron source realized by arranging a number of surface conduction electron-emitting devices that are cold-cathode type devices and an image-forming apparatus comprising such an electron source.
There have been known two types of electron-emitting device; the thermoelectron emission type and the cold cathode electron emission type. Of these, the cold cathode emission type refers to devices including field emission type (hereinafter referred to as the FE type) devices, metal/insulation layer/metal type (hereinafter referred to as the MIM type) electron-emitting devices and surface conduction electron-emitting devices.
Examples of FE type device include those proposed by W. P. Dyke & W. W. Dolan, "Field emission", Advance in Electron Physics, 8, 89 (1956) and C. A. Spindt, "PHYSICAL Properties of thin-film field emission cathodes with molybdenum cones", J. Appl. Phys., 47, 5248 (1976).
Examples of MIM device are disclosed in papers including C. A. Mead, "Operation of Tunnel-Emission Devices", J. Appl. Phys., 32, 646 (1961).
Examples of surface conduction electron-emitting device include one proposed by M. I. Elinson, Radio Eng. Electron Phys., 10, 1290 (1965).
A surface conduction electron-emitting device is realized by utilizing the phenomenon that electrons are emitted out of a small thin film formed on a substrate when an electric current is forced to flow in parallel with the film surface. While Elinson et al. proposes the use of SnO.sub.2 thin film for a device of this type, the use of Au thin film is proposed in G. Dittmer, "Thin Solid Films", 9, 317 (1972), whereas the use of In.sub.2 O.sub.3 /SnO.sub.2 and that of carbon thin film are discussed respectively in M. Hartwell and C. G. Fonstad, "IEEE Trans. ED Conf.", 519 (1975) and in H. Araki et al., "Vacuum", Vol. 26, No. 1, p. 22 (1983).
The applicant of the present patent application has made a number of proposals for surface conduction electron-emitting devices including the one schematically illustrated in FIGS. 2A and 2B. Since the configuration of such a surface conduction electron-emitting device, a method of manufacturing the same and an image-forming apparatus realized by using such devices are disclosed in Japanese Patent Application Laid-Open No. 7-235255, they will be described only summarily here. Referring to FIGS. 2A and 2B, the surface conduction electron-emitting device comprises a substrate 1, a pair of device electrodes 2 and 3 and an electroconductive film 4, which includes an electron-emitting region 5 as part thereof. With a method of producing an electron-emitting region 5, a part of the electroconductive film is deformed, transformed or destroyed to make it electrically highly resistive by applying a voltage to the paired device electrodes. This process is referred to as "energization forming process". In order to produce an electron-emitting region that operates well for electron emission in an electroconductive film, the latter preferably comprises electroconductive fine particles such as fine particles of palladium oxide (PdO). A pulse voltage is preferably used for an energization forming process. A pulse voltage to be used for energization forming may have a constant wave height as shown in FIG. 16A or, alternatively, it may have a gradually increasing wave height as shown in FIG. 16B.
It has also been reported by the applicant of the present patent application that a carbonaceous film that contains carbon as principal ingredient is deposited in and around the electron-emitting region to remarkably increase the rate of electron emission of the device with an activation process. An activation process is typically performed by repetitively applying an appropriate pulse voltage to the electron-emitting region in an atmosphere containing gaseous organic substances.
The carbonaceous film containing carbon or carbon compound as principal ingredient typically comprises graphite (including so-called HOPG, PG and GC, of which HOPG refers to graphite having a substantially perfect crystal structure, while PG and GC respectively refer to one having a somewhat irregular crystal structure with a crystal grain size of about 20 nm and one having a considerably irregular crystal structure with a grain size of about 2 nm) and/or non-crystalline carbon (including amorphous carbon and a mixture of amorphous carbon and graphite containing fine crystal grains).
FIG. 2C of the accompanying drawings schematically illustrates the electron-emitting region and its vicinity. The carbonaceous film may be deposited in a number of different ways depending on the pulse voltages applied to the electron-emitting region. If the pulse polarity is one-directional, a carbonaceous film containing carbon or carbon compound as principal ingredient is formed mainly on the high potential side of the fissure or fissures produced in the energization forming process (as a result of deformation or destruction). Note that, in FIG. 2C, the device electrode 3 represents the high potential side. Electrons are emitted from the fissure and its vicinity. A carbonaceous film may be deposited evenly on the opposite sides of the fissure by carrying out the activation process, frequently switching the polarity of the pulse voltage that is being applied.
Then, the electron-emitting device is preferably subjected to a process referred to as "stabilization process", where the molecules of the organic substances utilized in the activation process that have been adsorbed by the substrate of the electron-emitting device and the inner walls of the vacuum envelope of the image-forming apparatus comprising the device are removed in order to prevent the carbonaceous film containing carbon or carbon compound as principal ingredient from undesirably growing any further and make the device operate stably. More specifically, in a stabilization process, the device is placed and heated in a vacuum vessel while the latter is gradually evacuated by means of an exhaust system for producing ultra-high vacuum typically comprising a scroll pump and an ion pump so that consequently the organic substances remaining on the device are satisfactorily removed to prevent the deposited carbonaceous film from growing any further and make the device operate stably for electron emission.
The problems that arises when no stabilization process is performed include the following as specifically described in Japanese Patent Application Laid-Open No. 7-235275 filed also by the applicant of the present patent application as cited above.
(1) If the electron-emitting device is driven to operate after a long pause, it can show varied electric characteristics (particularly in terms of the current-voltage relationship) such that the emission current produced by the device temporarily grows remarkably.
(2) The emission current of the device changes significantly if the pulse width of the voltage being applied to the device is varied, and as a result, the quantity of electron emitted from the device is hardly controlled by controlling the pulse width.
(3) The electric characteristics of the device are varied by changing the pulse height of the voltage being applied to the device, and as a result, the quantity of electron emitted from the device hardly controlled by controlling the pulse height.
(4) If the device is used in an image-forming apparatus, the brightness and the colors of the image produced by the apparatus is hardly controlled as desired because of the above problems.
The above identified patent publication also discloses that the above problems are attributable to "fluctuations in the volume of organic molecules found in the vacuum atmosphere particularly on the surface and the surrounding areas of the electron-emitting device" so that "the device may be made to show a stable electron-emitting performance without fluctuations in the emission current and the device current by minimizing the partial pressure of organic molecules". It says specifically that the partial pressure of the organic substances in the vacuum vessel is preferably less than 1.3.times.10.sup.-6 Pa (1.times.10.sup.-8 Torr), more preferably less than 1.3.times.10.sup.-8 Pa (1.times.10.sup.-10 Torr). Additionally, the total pressure in the vacuum vessel is preferably less than 1.3.times.10.sup.-4 Pa, more preferably less than 1.3.times.10.sup.-5 Pa, and most preferably less than 1.3.times.10.sup.-6 Pa.
The above identified patent documents also describe a technique of applying a pulse voltage to the device in vacuum of about 10.sup.-2 -10.sup.-3 Pa (10.sup.-4 -10.sup.-5 Torr) for an activation process in order to deposit carbon and/or carbon compounds on the device out of the organic substances found in the vacuum. An electron-emitting device that has been subjected to an activation process either shows a performance that the device current If monotonously increases with the device voltage Vf (a property referred to as MI characteristic) or shows a performance characterized by a voltage-controlled-negative-resistance (a property referred to as VCNR characteristic) depending on the conditions of the activation process, the conditions where the performance is observed, and the like. An electron-emitting device with a VCNR characteristic can shift the property depending on the conditions where the characteristic is determined by measurement. More specifically, the electron-emitting device that originally shows a VCNR characteristic shows various characteristics depending on the sweeping rate of the device voltage at the time of measurement, the time period during which the device has been left unoperated before the measurement, the highest voltage applied to the device for the measurement and other factors. For instance, the device can become to show an MI characteristic if the sweeping rate is high, although it is made to show a VCNR characteristic again when the sweeping rate is reduced. While the device shows an MI characteristic for its emission current Ie in any event, the electron-emitting performance of the device remains unstable and varies depending on the conditions of measurement.
After a stabilization process conducted to avoid the above listed problems, the device shows a relationship between the device voltage and the device current that is unequivocally defined within an operating voltage range under a maximum voltage limit. In other words, the device comes to show a monotonously increasing characteristic (MI characteristic) so that the relationship between the device voltage and the emission current is also unequivocally defined to avoid the above listed problems.
Thus, as a result of the stabilization process for stabilizing the electron-emitting performance of the electron-emitting device, the organic substances used to produce the carbonaceous film containing carbon or carbon compound as principal ingredient are effectively removed. However, a problem arises on the electron-emitting device if the carbonaceous film containing carbon or carbon compound as principal ingredient is lost for some reason or other because the organic substances used to produce the carbonaceous film are already gone and hence the carbonaceous film cannot be restored. Additionally, the electron-emitting device can gradually lose the carbonaceous film that contains carbon as principal ingredient to degrade its electron-emitting performance particularly when it is operated continuously for a prolonged period of time. The carbonaceous film may be lost for a number of reasons including evaporation due to the electric field applied to the electron-emitting region, evaporation due to the Joule's heat generated by the device current and the etching effect of ions colliding with the carbonaceous film.